The present invention relates to an apparatus for continuously casting molten metal, particularly an apparatus for continuously producing a cast object having an improved quality by agitating with electromagnetic induction during the continuous casting.
It is usual for very many kinds of molten metals to contain a variety of foreign elements, for example carbon, silicon, manganese, phosphorus, sulfur, aluminum, nickel, chromium, etc. It is known that in the process of cooling a cast object contaning such elements as mentioned above to solidify it, as solidification progresses these elements concentrate in the center of the cast object so as to crystallize into simple substances or compounds. Thus, the part of the cast object that solidifies last may contain such elements in a greater amount than the other parts which have solidified earlier. Therefore, said part may solidify at the lowest temperature, and is known by the name "segregation part ".
Products made of the casting containing the segregation part tend to have a lowered and/or nonuniformly distributed mechanical strength, possible causing troubles at the time of welding, so that they have a low commercial value. In order to prevent such troubles, there has been devised in the case of traditional casting using a closed-end mold a raised rate of feed so that the increased feed will contain the segregation part which can be cut away afterward. However, such a method results in a lowered production yield and an increased production cost, constituting a disadvantage thereof. In the case of a continuous casting method, segregation takes place in the direction perpendicular to that of withdrawing the cast object to so great an extent that no good devices or operation conditions are available to reduce, without difficulty, the size of the segregation part below the level achieved in the case of using the abovementioned way of casting by using the closed-end mold.
In efforts to solve such problems as the lowered productioon yield in the case of casting by using the closed end mold and the nonuniform distrubution of quality of the casting made by the continuous casting method, there has been recognized the necessity of a molten metal cooling process which works sufficiently to halt the segregation of the elements contained in the molten metal for obtaining a uniform distribution thereof. There is known such a method of agitating the crater of the cast object by using an electromagnetic force. However, this method has not yet been industrialized, as there has not yet been developed the optimum agitating force and flow pattern thereof according to kinds of molten metals, even though it is considered to be the most stable among various methods.
It is confirmed that in the cast object solidifying process using the conventional electromagnetic agitation method, dendrite is broken up, but a heavier agitation for further breaking of the dendrite will cause the formation of the so-called "white band "on the border between the dendrite and the free crystal. In the case of the cast slab, the white band tends to be thick along the short side which the flow of agitated molten metal hits.
As for the white band, there has been no one investigate it, much less do research into its composition in relation to the quality of the cast object. So far as the inventors of the present invention have confirmed, the white band is a kind of negative segregation, the content of which is less concentrated than that of the other part, proving that such a cast object is not qualified as commercial material; and, the white band itself spoils the good appearance of a product made of such a cast object.
In whichever direction agitation occurs, it is unavoidable that the white band will be formed so long as the molten metal flows. According to the experiments conducted by the inventors of the present invention, it has been found to be impossible to prevent formation of the white band by modifications of the electromagnetic agitation applied to the object cast duriing continuous casting. Agitation according to the known method for preventing the formation of the white band fails to reduce the amount of dendrite, resulting in a failure also of the original purpose of the agitation.
As for the electromagnetic agitator for use with the continuous casting apparatus, its main body is heated to a high temperature during operation by radiant heat from the cast object and heat generated from current running in the coils. As a result, its frame deforms, making it difficult to supply current to the coils throughout a long continuous operation and also to reduce the gap between the cast object and the agitator to a minimum, causing such troubles as a serious loss of electromagnetic energy, a shorter life of the coils and flaws on the surface of the product.
In order to get rid of these problems, there has been used a device to cool the electromagnetic agitator with an appropriate coolant. Thus, the agitator has its main body cooled near or to room temperature for some time from the start of operation or at the time of repairs, and then it is heated, thus being subjected to cycles of heating and cooling. In such case, where a water cooling system, which is the most economical for this purpose, is used, water drops stay on the inside wall of the frame and the surface of the coils, causing such troubles as: (a) the interruption of current supply or limitation of applied voltage due to the degradation of the insulation on the coils; and (b) the production of rust on the parts of the agitator due to the high temperature inside the frame. The integration of current supply to the coils causes an integration in the operation of the electromagnetic agitator; and the production of rust necessitates extensive maintenance of the agitator. The part of the continuous casting apparatus on which the main body of the electromagnetic agitator is mounted is in a narrow place subjected to dust deposit and high temperature, such position making almost impossible daily inspection and maintenance in spite of the necessity thereof.
Such agitator is provided usually at an appropriate position away from the surface of the molten metal in the mold in the direction of withdrawing the cast object. In order to prevent such troubles in the operation as the formation of cracks inside the cast object or break-out (of molten out of the solidified shell), it is necessary to provide the agitator without changing the pitch of the cast object supporting guide rolls, a great number of which are provided in the direction of withdrawing the cast object in the continuous casting apparatus.